Transformer manufacturers control a magnetic core gap by linking physical construction to a measured inductance factor, commonly called AL value. The mechanical gap matters, but surface finish, spacer material, distributed gaps, core permeability, clamping force, assembly alignment, temperature, and test method all influence the final electrical result.
Gaps are used in flyback transformers, inductors, coupled inductors, current transformers, and other magnetic components that need controlled inductance or energy storage. A drawing that specifies only a nominal gap dimension may not guarantee the required inductance across material and production variation.
What AL value represents
AL value relates inductance to the square of turns under a defined low-signal condition. It is a practical way to characterize the effective magnetic reluctance of the assembled core. The value is only meaningful when frequency, test voltage or current, temperature, winding turns, fixture, and core clamping are controlled.
Mechanical gap and electrical gap are not identical
A center-leg grind, outer-leg spacer, or distributed gap creates different fringing fields and assembly behavior. Surface roughness and nonparallel faces can introduce local air regions. Adhesive thickness or a coating can add reluctance. Clamping pressure may change how the halves seat, especially with small gaps.
For this reason, production often controls the final AL value or component inductance rather than relying only on a caliper measurement. Mechanical dimensions remain important for process setup and traceability.
Fringing affects winding loss
Flux spreads around a discrete gap and can induce eddy currents in nearby conductors, shields, clamps, or PCB copper. Turns placed close to the gap may run hotter than a loss calculation predicts. Winding placement, conductor type, gap distribution, and nearby hardware should be evaluated together during prototype thermal testing.
Build a realistic tolerance chain
The final inductance includes core AL variation, turns count, winding placement, gap tolerance, assembly pressure, temperature, and DC bias. Tightening only the mechanical gap may add cost without controlling the dominant source. The customer and transformer manufacturer should identify which operating characteristic actually needs the tight range.
Production control methods
- Approved core material, geometry, finish, and supplier
- Controlled grinding, spacer, or distributed-gap process
- Fixture alignment and clamping-force definition
- AL or inductance measurement at stated frequency and signal level
- Sampling of gap dimension or spacer thickness
- Core-pair and material-lot traceability when risk requires it
- Clear handling criteria for chipped or contaminated mating surfaces
BaoHui Tech uses the component drawing and electrical test specification to connect core-gap construction with final inductance. Customers requesting a custom magnetic design should provide inductance-versus-current requirements, temperature range, ripple waveform, energy storage, and acceptable tolerance rather than only a nominal gap.
Frequently asked questions
Can AL value predict inductance under DC bias?
Not by itself. Low-signal AL is a useful baseline, but the material, gap, flux density, temperature, and winding determine inductance under operating bias.
Why can two cores with the same measured gap have different AL values?
Material permeability, surface flatness, coatings, seating, distributed parasitic gaps, and measurement conditions can differ.
Should the core gap be measured on every unit?
Often the electrical inductance test is more practical for every unit, with dimensional gap checks used for setup or sampling. The control plan should match risk and process capability.